This invention relates to methods and systems for determining depth distribution of radiation-emitting material located in a source medium and radiation detector system for use therein.
In principle, in situ gamma-ray spectrometry determines the quantities of radionuclides in some source medium with a portable detector. In comparison, the more established method of laboratory gamma-ray spectroscopy consists of taking small samples of the medium into the laboratory for gamma-ray analysis. In situ gamma-ray spectrometry characterizes a larger volume of material, requires less time to determine accurate radionuclide concentrations, and minimizes worker doses and the risk of radioactive contamination. The main limitation of in situ gamma-ray spectrometry lies in determining the depth distribution of radionuclides.
In general, radionuclide depth distributions aid conventional in situ gamma-ray spectrometry in determining accurate radionuclide inventories and surface does rates from individual radionuclides. Depth distributions also represent reliable data for radionuclide transport studies. Indications of neutron or energetic charged particle fluxes can result from determinations of the activation as a function of material depth. For decontamination and decommissioning activities, the radionuclide depth distribution determines the amount of material that must be remediated to satisfy the release limits.
To date, three in situ gamma-ray spectroscopic methods have been used to determine the depth distribution of radionuclides in soil and are presented hereinbelow. These three in situ methods are based on multiple photopeak responses, the photopeak-to-valley ratio, and the attenuation of a lead plate as illustrated in FIGS. 1a and 1b. Each method requires a priori assumptions of the depth distribution function and uses a gamma-ray spectrometer. Spectrometers allow the users to decipher the energies of gamma-ray emissions, a necessity for determining the specific radioisotope present. In addition to usually assuming a uniform soil density with depth, all three approaches for determining depth distributions also assume a spatially uniform radionuclide distribution. All three in situ methods require a priori assumptions of the functional form for the depth distribution. The multiple photopeak and peak-to-valley methods only have the ability of determining a single depth parameter. An exception exists if the radionuclide of interest emits three or more significant gamma-rays, decently separated in energy, and if the spectrometer used has sufficient energy resolution to identify and separate each gamma-ray emission. In such cases, the multiple photopeak method could determine one fewer number of depth parameters than the number of significant gamma-rays emissions. The subsurface maxima exhibited by aged 137Cs fallout in soil are best described by at least two depth parameters and can not be adequately characterized by a single depth parameter. Table 1 summarizes the advantages and disadvantages of the three in situ methods.
In addition to the three in situ methods for determining depth distributions, spectroscopic measurements in boreholes have also been studied for applications in oil wells. Because boring itself qualifies as an invasive process, borehole measurements should be considered a quasi-in-situ approach. In addition to increased contamination risks, borehole measurements require boring equipment and custom fabricated detection equipment (extended cryostat lengths for HPGe detectors).
Three other imaging techniques include: pinhole collimation, coded aperture imaging, and Compton scatter imaging. The main limitation, common to all three of these imaging techniques, is the energy resolution of the detectors used. These other imaging techniques utilize position-sensitive detector arrays, which typically are large scintillation crystals with insufficient energy resolution for complex gamma-ray fields. For characterizing low levels of radioactivity, advancements in position-sensitive semiconductor detectors have not yet yielded devices that are large enough for adequate sensitivities or affordable enough for a rugged and portable in situ system.
U.S. Pat. No. 4,197,460 to Anger discloses a collimator assembly used to perform multi-angle nuclear imaging and the results are used to estimate relative depth of objects. Multi-angle display circuits divide the probe radiation image into different regions.
U.S. Pat. No. 3,979,594 to Anger discloses how relative positions of radiation sources at different depths are estimated via a focused collimator. Multiple-channel collimators are mentioned as an option to be used.
U.S. Pat. No. 5,429,135 to Hawman et al. discloses how a focusing collimator detects the depth of an organ in nuclear medicine.
U.S. Pat. No. 5,442,180 to Perkins et al. discloses an apparatus for determining the concentration of radioactive constituents in test samples (such as surface soil) by means of a real-time direct readout.
Other U.S. patents of a more general interest include: U.S. Pat. Nos. 4,394,576; 5,773,829; and 5,870,191.
The primary measurement problem which is not solved by the prior art is the in situ determination of the depth distribution of gamma-ray emitting radionuclides in source media. Contaminated soil and activated concrete are common examples of anthropogenic radionuclides in large area geometries. For these measurement situations, the gamma-ray spectrum tends to be complex due to the presence of multiple-radionuclides (natural or anthropogenic in origin). Therefore, the spectrometers used in the field must possess excellent energy resolution to minimize the deleterious effects of interfering gamma-ray emissions. Other practical issues are that an in situ detection system should be portable and rugged. Because it is not uncommon for low levels of anthropogenic radionuclides to be present in smaller quantities than natural radionuclides, it is important that the detection system also possess a sufficient gamma-ray detection efficiency for reasonable counting times.
An object of the present invention is to provide a method and system for determining depth distribution of radiation-emitting material located in a source medium and a radiation detector system for use therein wherein the invention can be used with respect to any source medium as long as its attenuation properties are insignificant, known, measurable, or estimable in any way.
Another object of the present invention is to provide a method and system for determining depth distribution of radiation-emitting material located in a source medium and a radiation detector system for use therein wherein in situ radiation measurements are performed such as gamma-ray spectrometry and offer a superior ability for characterizing complex depth distributions.
Still another object of the present invention is to provide a method and system for determining depth distribution of radiation-emitting material located in a source medium and a radiation detector system for use therein wherein the radiation-emitting material is radionuclides.
Yet still another object of the present invention is to provide a method and system for determining depth distribution of radiation-emitting material located in a source medium and a radiation detector system for use therein wherein conventional radiation detection equipment can be employed.
Yet still another object of the present invention is to provide a method and system for determining depth distribution of radiation-emitting material located in a source medium and a radiation detector system for use therein wherein the depth distribution is calculated without a priori knowledge about the depth distributions without required a priori selection of a specific functional form for the depth distribution and without the need for invasive core samplings.
In carrying out the above objects and other objects of the present invention, a method for determining depth distribution of radiation-emitting material located in a source medium is provided. The method includes detecting radiation emitted by the material within selected ranges of polar angles relative to a detector axis which is substantially perpendicular to an outer surface of the source medium to produce a plurality of corresponding electrical signals. The method also includes processing the plurality of electrical signals to obtain the depth distribution of the radiation-emitting material in the source medium.
The radiation may include x-ray emissions and/or gamma-ray emissions.
The material may be radionuclides or is energized so that the material emits the radiation.
The source medium may be soil or building materials such as concrete and/or steel.
The source medium may be an airborne plume.
The step of detecting is preferably at least partially performed with a detector having intrinsic efficiency and angular response and wherein the step of processing processes data representing the intrinsic efficiency and the angular response with the electrical signals to obtain the depth distribution.
In further carrying out the above objects and other objects of the present invention, a system for determining depth distribution of radiation-emitting material located in a source medium is provided. The system includes at least one detector assembly for detecting radiation emitted by the material within selected ranges of polar angles relative to a detector axis which is substantially perpendicular to an outer surface of the source medium to produce a plurality of corresponding electrical signals. The system also includes a signal processor for processing the plurality of electrical signals to obtain the depth distribution of the radiation-emitting material in the source medium.
The detector assembly may include a radiation detector and a radiation shield which surrounds the radiation detector, wherein the at least one detector assembly is adjustable to allow the radiation detector to detect radiation within the selected ranges of the polar angles and the radiation shield substantially blocks radiation outside the selected ranges of the polar angles.
The shield may be a collimator which is cylindrically symmetrical.
The collimator may include a plurality of collimator pieces which can be assembled into a plurality of geometric arrangements corresponding to the ranges of polar angles.
The collimator may be adjustable into a plurality of geometric arrangements corresponding to the ranges of polar angles.
The at least one detector assembly may be a radiation spectrometer.
The at least one detector assembly may include at least one of an array of detectors, a position-sensitive detector and a scanning detector.
The collimator may be movable relative to the detector.
For example, the collimator may include at least one collimator piece which is pivotally movable relative to the detector.
A pair of detector assemblies allow the system to focus at a selected depth of the source medium.
The collimator may be linearly movable relative to the detector.
The collimator may be cylindrically symmetrical about the detector.
The at least one detector assembly could have a relatively narrow field of view that is capable of being directed at a desired polar angle for a measurement and is rotatable about the detector axis. The same effect can also be accomplished with rotating the entire detector assembly, with a narrow field of view and at a fixed-polar angle, about the normal of the source surface.
Still further in carrying out the above objects and other objects of the present invention, a radiation detector system is provided. The system includes at least one central radiation detector for converting ionizing radiation into a first signal. The system also includes at least one satellite radiation detector positioned adjacent the at least one central radiation detector for converting ionizing radiation into at least one second signal. The system further includes at least one radiation shield disposed adjacent the at least one satellite radiation detector to substantially block ionizing radiation originating outside a field of view of the at least one satellite radiation detector. The first signal and the at least one second signal represent a spectral fingerprint of an area and spatial distribution of an ionizing radiation source within the area.
The satellite radiation detectors may be spectroscopic radiation detectors.
The ionizing radiation may include gamma rays wherein the system is a position-sensitive, compound gamma ray spectrometer.
The central radiation detector preferably includes a semiconductor substrate.
Radionuclides represent one of the most significant contamination problems for the Department of Energy (DOE). Implementation of this invention would decrease the risk of the public and workers to radiation and significantly reduce the cost and time of radiation characterization activities. As a powerful tool for radionuclide characterization and verification of remediation, this invention is immediately applicable to the widespread radionuclide cleanup activities of soil contamination as well as activated or contaminated building materials (such as concrete or steel) across the DOE complex and commercial nuclear power industry. This invention could also determine vertical or horizontal distributions of radionuclides in airborne plumes or be applied to borehole measurements for determining radionuclide depth distributions. Modifications could be made for the characterization of contamination in laboratories and for other geometries, such as tanks, drums, pipes, etc.
Conventional in situ gamma-ray spectrometry uses an unshielded gamma-ray detector placed 1 m above the soil surface. This invention implements a unique collimator with conventional or unconventional radiation detection equipment. A cylindrically symmetric collimator is positioned so that its axis is normal to surface of the area source and surrounds the detector and allows only those gamma-rays emitted from a selected range of polar angles (measured off the detector axis) to be detected. Adjustment of the collimator enables the detection of gamma-rays emitted from a different range of polar angles and preferential depths. Assuming a spatially uniform radionuclide distribution (in the plane normal to the collimator axis) within each depth increment and any radionuclide depth distribution (uniform or otherwise) within each depth increment, the unattenuated or uncollided gamma-ray flux from each depth increment can be calculated and paired with the intrinsic efficiency and angular response of the detector to yield a detector response matrix over the selected depth increments and range of measured polar angles. The uncollided or unattenuated gamma-rays are those gamma-rays which are emitted from a radionuclide and do not interact in the material between the radionuclide and the detector.
The present invention is an improvement over the lead plate method of FIGS. 1a and 1b. By avoiding the summing effect of large and small polar angles from moving a simple lead plate farther from the detector, the present invention exhibits a smaller measurement error. In addition, the present invention""s angled edges allow for more effective shielding of those radiations emitted outside of the polar angle range and allow for a more straightforward response calculation.
This invention offers superior sensitivity, easier operation, and greater robustness requiring less maintenance under rugged conditions as compared with the prior art.
Each measurement of a particular range of polar angles is performed with a different combination of the collimator pieces or by setting a single adjustable collimator to a similar geometry. The matrix equation for determining the detector response for each collimator setup could take the following form:
m=Hd 
where m is column vector for the detector response or measured photopeak count rate at each range of polar angles, H is the detector response matrix for each combination of polar angle and depth increment and represents the photopeak count rate from a unit radionuclide specific activity in a specific depth increment for a specific collimator setup, and d is a column vector for the radionuclide depth distribution (radionuclide specific activity in units of Bq per gram of material in each depth increment for example). Calculation of the depth distribution follows from solution of the following matrix equation:
d=H+m 
where H+ is the matrix such that H+m=H+Hd=d (note that H+ is called the inverse of the detector response matrix, H, when H is a square matrix such that the column vectors m and d are of the same dimension). Therefore, processing of the measured angular data in m yields a reconstruction of the radionuclide distribution with depth, d.
It should be noted that improvements in the depth distribution determinations could be obtained from (a) increasing the collimated measurements to invoke an overdetermined situation (oversampled method), and (b) from simplifying the response matrix by neglecting the elements of the matrix that are small with respect to the other elements for a particular polar angle measurement. In addition to analytical calculations and actual measurements, the response matrix can also be computed from Monte Carlo computer simulations.
The elements of the invention which are new compared to the current prior art are:
defining independent spatially uniform depth increments each with a uniform or any other depth radionuclide distribution within each depth increment;
using a cylindrically symmetric collimator to allow significant detector photopeak response over a small range of polar angles and negligible photopeak response for gamma-rays incident on the detector at polar angle outside of the small range; and
determining the detector response for a small range of polar angles from a unit radionuclide source in each depth increment (defining the detector response matrix).
Instead of using a single gamma-ray spectrometer with several different collimator setups, simultaneous angular measurements could be performed with a position sensitive detector, array of detectors, or scanning detectors. Instead of several fixed collimator pieces, a single mechanically adjustable collimator could achieve the different collimator setups.